1. Field of the Invention
The present invention relates to a method and apparatus for measuring a thickness of a thin film, using an electron beam.
2. Description of the Related Art
Hitherto, for example, JP-A-H06-273297 discloses a method wherein, upon forming a sample into a thin film by ion beam irradiation, an electron beam is applied to the sample simultaneously with an ion beam so as to prevent excessive etching thereof by detecting the electron beam transmitted through the sample with a Faraday cup.
Similarly, JP-A-H08-5528 discloses a method wherein, when an ion beam processing apparatus is used for preparing a sample for a transmission electron microscope, the ion beam processing amount is controlled by applying an electron beam to a processing portion to detect a current amount of the electron beam transmitted through the processing portion.
According to the methods described in JP-A-H06-273297 and JP-A-H08-5528, however, inasmuch as the amount of the electron beam transmitted through the sample is measured, it is necessary that the sample subjected to measurement be processed to be thin for allowing the electron beam to pass therethrough. Therefore, it has been difficult to measure a thickness of a thin film formed on a support substrate like in case of a normal semiconductor device.
Further, as described in, for example, JP-A-S63-9807, there has been known a method wherein an electron beam is applied to a thin film to collect secondary electrons emitted from the inside of the thin film and, based on a correlation between an amount of collected secondary electrons and a thickness of a thin film, the thickness of the thin film on a substrate is measured. In this method, however, it has been difficult to precisely collect secondary electrons emitted from a thin film formed at the bottom of a hole with a high aspect ratio.
For solving the foregoing problems of the prior art, the present inventor has succeeded in developing a technique wherein the value of substrate current that flows in a substrate upon applying an electron beam to a thin film formed on the substrate is measured, and the thickness of the thin film is calculated based on reference data, and has filed a patent application (JP-A-2000-180143). In this method, since the substrate current value is measured directly from the substrate, i.e. not measuring the amount of the electron beam transmitted through the sample, it is possible to measure even the thickness of a thin film formed on the substrate.
FIG. 1 is a block diagram showing a film thickness measuring apparatus disclosed in JP-A-2000-180143. This apparatus comprises an electron gun 3 for radiating an electron beam to a thin film 2 on a substrate 1, an electrode 4 disposed in contact with the underside of the substrate 1, and a current measuring section 5 for measuring the value of substrate current collected to the electrode 4. The current measured at the current measuring section 5 is adjusted through a current amplifier 6 and a differential amplifier 7 and converted into a digital signal by an A/D converter 9. The film thickness measuring apparatus further comprises a measured current storing section 10 for storing a measured current value converted into a digital signal, an analytical curve data storing section 11 for storing analytical curve data measured using available standard samples, and an analytical curve data comparing section 12 for comparing the analytical curve data and the measured current value.
According to the film thickness measuring apparatus thus configured, there is an effect that the thickness of a thin film, particularly an extremely thin film, can be accurately measured.
The invention of JP-A-2000-180143 employs the following principle. When an electron beam with low energy ranging approximately from several hundred eV to several keV is applied to a sample, secondary electrons are emitted from the neighborhood of the surface of the sample. In general, the secondary electron emission capability of conductors or semiconductors is small, while that of insulators is large. For example, the secondary electron emission capability of silicon being a semiconductor is approximately 0.9, while that of a silicon oxide film being an insulator is approximately 2.
Accordingly, when an electron beam is applied to a semiconductor device with a silicon oxide thin film formed on the surface of a silicon substrate, more secondary electrons are emitted from the silicon oxide film. In this event, electrons flow out from the silicon substrate into the silicon oxide film for compensating for the secondary electrons emitted from the silicon oxide film. That is, the substrate current that is the sum of a current generated by the applied electron beam and a compensation current in the direction opposite to the direction of the generated current flows in the silicon substrate.
FIGS. 2A and 2B are an exemplary diagram showing this principle. As shown in FIG. 2A, in case a silicon oxide thin film is formed on a silicon substrate, when one electron is applied thereto by means of an electron beam, two electrons are emitted from the silicon oxide film as secondary electrons. This results in that one electron is lost from the silicon oxide film, so that one electron flows out from the silicon substrate into the silicon oxide film for compensating for the lost electron. In this event, a substrate current in the direction opposite to the direction of a current generated by the electron beam flows in the silicon substrate.
On the other hand, as shown in FIG. 2B, in case no silicon oxide film is formed on a silicon substrate, when one electron is applied thereto by means of an electron beam, 0.9 electron is emitted from the silicon substrate as secondary electron. As a result, a substrate current corresponding to an amount obtained by subtracting an emitted electron amount from an applied electron amount flows in the silicon substrate in the direction of a current generated by the electron beam.
As described above, when the silicon oxide film is not formed on the silicon substrate, the secondary electron emission amount is small and thus the current generated by the electron beam is dominant, while, as the thickness of the silicon oxide film increases, the compensation current increases. Therefore, by deriving in advance reference data showing a correlation between film thicknesses and substrate current values with respect to standard samples and comparing a measured substrate current value with the reference data, the thickness of a thin film can be calculated.
However, if the film thickness measuring method described in JP-A-2000-180143 is applied to the measurement of a thickness of a thin film formed at the bottom of a hole with a high aspect ratio, a portion of secondary electrons emitted from the thin film is accumulated on the wall of the hole as shown in FIG. 3. As a result, an electric field is generated in the hole due to the secondary electrons accumulated on the wall thereof. It has been found out that further emission of secondary electrons from the surface of the thin film is suppressed due to an influence of this electric field, so that even if the thicknesses are equal to each other, substrate current values differ from each other between a thin film formed on the flat surface and a thin film formed at the bottom of the hole. Specifically, the amount of the secondary electrons emitted from the thin film formed at the bottom of the hole is reduced and thus the compensation current is resultantly reduced, therefore, there arises a problem wherein assuming that the direction of the current generated by the electron beam is a positive direction, the substrate current value is deviated in the positive direction as compared with the flat surface.
The foregoing problem is peculiar to the case where the method described in JP-A-2000-180143 is employed in measuring the thickness of not only a thin film formed on a flat surface, but also a thin film formed at the bottom of a hole with a high aspect ratio or on a substrate having a concave-convex surface. The present invention aims to improve the technique described in JP-A-2000-180143 for solving the foregoing problem.
The present invention has been made under these circumstances and has an object to provide a technique for measuring a thickness of a thin film formed on a substrate and, in particular, a technique for precisely measuring a thickness of even a thin film formed at the bottom of a hole with a high aspect ratio or on a substrate having a concave-convex surface.
According to the present invention, there is provided a method of measuring a thickness of a thin film as a measurement object using a value of substrate current that flows in a substrate upon applying an electron beam to the thin film formed on the substrate. The method comprises the steps of acquiring reference data representing a correlation between film thicknesses and substrate current values with respect to standard samples; acquiring a value of substrate current that flows in the substrate upon applying an electron beam to the thin film as the measurement object formed on the substrate; correcting the substrate current value taking into account an influence of a charge distribution generated in the neighborhood of the thin film due to the application of the electron beam; and calculating the thickness of the thin film as the measurement object based on the corrected substrate current value taking into account the reference data.
According to this method, since the substrate current value is corrected taking into account an influence of a charge distribution of secondary electrons, even if the secondary electrons emitted from the thin film upon applying the electron beam to the thin film are accumulated in the neighborhood of the thin film, the substrate current value can be compared with the reference data by removing such an influence. Therefore, for example, the thickness of even a thin film formed at the bottom of a hole with a high aspect ratio can be measured accurately.
The substrate current is the sum of a current generated by the electron beam applied to the substrate and a compensation current representing an amount of electrons that flow out from the substrate into the thin film for compensating for secondary electrons emitted from the thin film by applying the electron beam to the thin film.
The substrate current value may be raw data obtained by measuring a value of substrate current that flows in the substrate upon applying the electron beam to the thin film as the measurement object, or may be data obtained by adjusting the raw data through amplification or the like, or may be data obtained by converting those data into digital signals, meaning that any data will do inasmuch as the data is derived from the measured substrate current value.
The energy (acceleration voltage) of the electron beam used in this method is preferably up to the extent where the electron beam does not penetrate the substrate, i.e. for example, up to approximately 10 keV.
The reference data may represent a correlation between a value of substrate current for reference that should flow in the substrate upon applying an electron beam to a thin film with a given thickness, and this given film thickness.
According to the present invention, there is provided a method of measuring a thickness of a thin film as a measurement object using a value of substrate current that flows in a substrate upon applying an electron beam to the thin film formed on the substrate. The method comprises the steps of acquiring reference data representing a correlation between film thicknesses and substrate current values with respect to standard samples; acquiring a value of substrate current that flows in the substrate upon applying an electron beam to the thin film as the measurement object formed on the substrate; correcting the substrate current value taking into account an influence of a configuration of the surface of the substrate in the neighborhood of the thin film; and calculating the thickness of the thin film as the measurement object based on the corrected substrate current value taking into account the reference data.
According to this method, since the substrate current value is corrected taking into account an accumulated amount of secondary electrons depending on a configuration of the surface of the substrate in the neighborhood of the thin film, even if the secondary electrons emitted from the thin film upon applying the electron beam to the thin film are accumulated in the neighborhood of the thin film, the substrate current value can be compared with the reference data by removing such an influence. Therefore, for example, the thickness of even a thin film formed at the bottom of a hole with a high aspect ratio can be measured accurately.
The influence of the configuration of the surface of the substrate may be an influence caused by a difference in level between a region where the thin film is formed and the neighborhood thereof, or may be an influence caused by unevenness between a region where the thin film is formed and the neighborhood thereof.
The thin film may be provided at the bottom of a concave portion formed on an accumulation film on the substrate, and the correcting step may calculate an influence of a charge distribution depending on a configuration of the concave portion where the thin film is provided.
It may be arranged that the method further comprises a step of acquiring layout data representing the arrangement of the concave portions formed on the surface of the substrate, that the step of acquiring the substrate current value acquires positions on the thin films where the electron beam is applied, correspondingly to the substrate current values, and that the correcting step corrects the substrate current values based on the layout data of the positions where the electron beam is applied.
The substrate in the present invention represents a base material serving as a ground on which a thin film as a film thickness measurement object is formed, and further represents a semiconductor substrate with or without an accumulation film formed thereon, an insulating substrate with or without an accumulation film formed thereon, or the like. For example, it represents a silicon substrate with an insulating film formed thereon.
It may be arranged that the step of acquiring the layout data includes the steps of applying an electron beam to the thin films for acquiring the layout data, before applying the electron beam to the thin films for calculating the thicknesses of the thin films; acquiring a value of substrate current that flows in the substrate thereupon, correspondingly to each applying position of the electron beam; and detecting the arrangement of the concave portions formed on the surface of the substrate based on the substrate current values and the applying positions.
It may be arranged that the step of acquiring the layout data acquires the layout data based on design data.
It may be arranged that each concave portion is a hole, and the correcting step corrects the substrate current values using a correction equation having a radius of the hole as a variable.
It may be arranged that a plurality of concave portions are formed on the surface of the substrate, the step of acquiring the substrate current value acquires substrate current values correspondingly to positions of the concave portions, respectively, the step of acquiring layout data includes a step of detecting configurations of the concave potions correspondingly to the positions of the concave portions, respectively, and the correcting step corrects the substrate current values depending on the configurations of the concave portions, respectively.
According to the present invention, there is provided an apparatus for measuring a thickness of a thin film as a measurement object using a value of substrate current that flows in a substrate upon applying an electron beam to the thin film formed on the substrate. This apparatus comprises a reference data acquiring section for acquiring reference data representing a correlation between film thicknesses and substrate current values with respect to standard samples; a measured data acquiring section for acquiring a value of substrate current that flows in the substrate upon applying an electron beam to the thin film as the measurement object formed on the substrate; a correction processing section for correcting the substrate current value taking into account an influence of a charge distribution generated in the neighborhood of the thin film due to the application of the electron beam; and a calculation processing section for calculating the thickness of the thin film as the measurement object based on the corrected substrate current value taking into account the reference data.
According to this apparatus, since the correction processing section corrects the substrate current value taking into account an influence of a charge distribution of secondary electrons, even if the secondary electrons emitted from the thin film upon applying the electron beam to the thin film are accumulated in the neighborhood of the thin film, the calculation processing section can compare the substrate current value with the reference data by removing such an influence. Therefore, for example, the thickness of even a thin film formed at the bottom of a hole with a high aspect ratio can be measured accurately.
It may be arranged that the thin film is provided at the bottom of a concave portion formed on the surface of the substrate, and the correction processing section calculates the influence of the charge distribution depending on a configuration of the concave portion where the thin film is provided.
According to the present invention, there is provided an apparatus for measuring a thickness of a thin film as a measurement object using a value of substrate current that flows in a substrate upon applying an electron beam to the thin film formed on the substrate. This apparatus comprises a reference data acquiring section for acquiring reference data representing a correlation between film thicknesses and substrate current values with respect to standard samples; a measured data acquiring section for acquiring a value of substrate current that flows in the substrate upon applying an electron beam to the thin film as the measurement object formed on the substrate; a correction processing section for correcting the substrate current value taking into account an influence of a configuration of the surface of the substrate in the neighborhood of the thin film; and a calculation processing section for calculating the thickness of the thin film as the measurement object based on the corrected substrate current value taking into account the reference data.
According to this apparatus, since the correction processing section corrects the substrate current value taking into account an accumulated amount of secondary electrons depending on a configuration of the surface of the substrate in the neighborhood of the thin film, even if the secondary electrons emitted from the thin film upon applying the electron beam to the thin film are accumulated in the neighborhood of the thin film, the calculation processing section can compare the substrate current value with the reference data by removing such an influence. Therefore, for example, the thickness of even a thin film formed at the bottom of a hole with a high aspect ratio can be measured accurately.
It may be arranged that the apparatus further comprises an electron beam applying section for applying the electron beam to the thin film as the measurement object; and a current measuring section for measuring a value of substrate current that flows in the substrate upon applying the electron beam to the thin film, and that the measured data acquiring section acquires the substrate current value from the current measuring section.
It may be arranged that the current measuring section comprises an electrode provided in contact with the substrate, and measures a current flowing in the electrode as the substrate current value.
It may be arranged that the apparatus further comprises a layout data storing section for storing layout data representing the arrangement of the concave portion formed on the surface of the substrate, that the current measuring section acquires a position on the thin film where the electron beam is applied, correspondingly to the substrate current value, and that the correction processing section corrects the substrate current value based on the layout data of the position where the electron beam is applied.
It may be arranged that the concave portion is a hole, and the correction processing section corrects the substrate current value using a correction equation having a radius of the hole as a variable.
It may be arranged that a plurality of concave portions are formed on the surface of the substrate, that the measured data recording section stores substrate current values correspondingly to positions of the concave portions, respectively, that the layout data storing section stores configurations of the concave potions correspondingly to the positions of the concave portions, respectively, and that the correction processing section corrects the substrate current values depending on the configurations of the concave portions, respectively.
According to the present invention, there is provided a program for causing a computer to execute a method of processing of measuring a thickness of a thin film as a measurement object using a value of substrate current that flows in a substrate upon applying an electron beam to the thin film formed on the substrate. This program causes the computer to execute the method comprising the steps of: acquiring reference data representing a correlation between film thicknesses and substrate current values with respect to standard samples; acquiring a value of substrate current that flows in the substrate upon applying an electron beam to the thin film as the measurement object formed on the substrate; correcting the substrate current value taking into account an influence of a charge distribution generated in the neighborhood of the thin film due to the application of the electron beam; and calculating the thickness of the thin film as the measurement object based on the corrected substrate current value taking into account the reference data.
According to the present invention, there is provided a program for causing a computer to execute a method of processing of measuring a thickness of a thin film as a measurement object using a value of substrate current that flows in a substrate upon applying an electron beam to the thin film formed on the substrate. This program controls an information processing apparatus comprising: reference data acquiring means for acquiring reference data representing a correlation between film thicknesses and substrate current values with respect to standard samples; measured data acquiring means for acquiring a value of substrate current that flows in the substrate upon applying an electron beam to the thin film as the measurement object formed on the substrate; correction processing means for correcting the substrate current value; and calculation processing means for calculating the thickness of the thin film as the measurement object based on the corrected substrate current value taking into account the reference data, and causes the reference data acquiring means to execute a process of acquiring the reference data, causes the measured data acquiring means to execute a process of acquiring the substrate current value, causes the correction processing means to execute a process of correcting the substrate current value taking into account an influence of a charge distribution generated in the neighborhood of the thin film due to the application of the electron beam, and causes the calculation processing means to execute a process of reading out the reference data from the reference data storing means and calculating the thickness of the thin film as the measurement object based on the corrected substrate current value taking into account the reference data.
According to the present invention, there is provided a program for causing a computer to execute a method of processing of measuring a thickness of a thin film as a measurement object using a value of substrate current that flows in a substrate upon applying an electron beam to the thin film formed on the substrate. This program causes the computer to execute the method comprising the steps of: acquiring reference data representing a correlation between film thicknesses and substrate current values with respect to standard samples; acquiring a value of substrate current that flows in the substrate upon applying an electron beam to the thin film as the measurement object formed on the substrate; correcting the substrate current value taking into account an influence of a configuration of the surface of the substrate in the neighborhood of the thin film; and calculating the thickness of the thin film as the measurement object based on the corrected substrate current value taking into account the reference data.
According to the present invention, there is provided a program for causing a computer to execute a method of processing of measuring a thickness of a thin film as a measurement object using a value of substrate current that flows in a substrate upon applying an electron beam to the thin film formed on the substrate. This program controls an information processing apparatus comprising: reference data acquiring means for acquiring reference data representing a correlation between film thicknesses and substrate current values with respect to standard samples; measured data acquiring means for acquiring a value of substrate current that flows in the substrate upon applying an electron beam to the thin film as the measurement object formed on the substrate; correction processing means for correcting the substrate current value; and calculation processing means for calculating the thickness of the thin film as the measurement object based on the corrected substrate current value taking into account the reference data, and causes the reference data acquiring means to execute a process of acquiring the reference data, causes the measured data acquiring means to execute a process of acquiring the substrate current value, causes the correction processing means to execute a process of correcting the substrate current value taking into account an influence of a configuration of the surface of the substrate in the neighborhood of the thin film, and causes the calculation processing means to execute a process of reading out the reference data from the reference data storing means and calculating the thickness of the thin film as the measurement object based on the corrected substrate current value taking into account the reference data.
According to the present invention, there is provided a computer-readable recording medium recording a program for causing a computer to execute a method of processing of measuring a thickness of a thin film as a measurement object using a value of substrate current that flows in a substrate upon applying an electron beam to the thin film formed on the substrate. This recording medium records a program that causes the computer to execute the method comprising the steps of: acquiring reference data representing a correlation between film thicknesses and substrate current values with respect to standard samples; acquiring a value of substrate current that flows in the substrate upon applying an electron beam to the thin film as the measurement object formed on the substrate; correcting the substrate current value taking into account an influence of a charge distribution generated in the neighborhood of the thin film due to the application of the electron beam; and calculating the thickness of the thin film as the measurement object based on the corrected substrate current value taking into account the reference data.
According to the present invention, there is provided a computer-readable recording medium recording a program for causing a computer to execute a method of processing of measuring a thickness of a thin film as a measurement object using a value of substrate current that flows in a substrate upon applying an electron beam to the thin film formed on the substrate. This recording medium records a program that causes the computer to execute the method comprising the steps of: acquiring reference data representing a correlation between film thicknesses and substrate current values with respect to standard samples; acquiring a value of substrate current that flows in the substrate upon applying an electron beam to the thin film as the measurement object formed on the substrate; correcting the substrate current value taking into account an influence of a configuration of the surface of the substrate in the neighborhood of the thin film; and calculating the thickness of the thin film as the measurement object based on the corrected substrate current value taking into account the reference data.
The present invention is further applicable to desired combinations of the foregoing constituent elements, and conversion of expressions of the present invention among methods, apparatuses, systems, recording mediums, computer programs and so forth.